1. Field of the Invention
The field of the invention is that of bipolar transistors and especially vertical structure heterojunction bipolar transistors in which the current is conveyed perpendicularly to the surface of the substrate. These transistors are used for microwave applications.
A bipolar transistor is constituted by a stack of layers of semiconductor materials, of which at least one surface layer is etched in a mesa formation, namely a raised or embossed pattern. FIG. 1 illustrates a standard structure of a heterojunction bipolar transistor or HBT comprising a substrate S, a sub-collector SC, a collector C, a base B and an emitter E. Conventionally, a contact CE is made on the surface of the emitter by the superimposition of matched layers. Laterally, two contacts CB.sub.1 and CB.sub.2 are also made in the collector on either side of the base.
This type of vertical structure raises a problem of electron/hole recombination at the free surfaces S.sub.1 and S.sub.2 shown in FIG. 1. These recombinations play a major role in the deterioration of current gain.
The smaller the size of the transistors, the greater is this phenomenon, with the surface recombination phenomena playing a greater role. Consequently, the microwave applications of such structures are heavily penalized by this problem. Indeed, for microwave operation, a bipolar transistor consists of several elementary transistors (also called fingers) positioned in parallel. To limit the base resistance, it is necessary to limit the width of the emitter and therefore the width of each finger. Typically, the width of a finger may be in the range of 2 .mu.m for applications using frequencies below 100 GHz while the length of the finger of the emitter may be about 30 microns.
2. Description of the Prior Art
Certain solutions have already been brought to bear on this problem, especially in the context of HBT transistors made with GaAlAs/GaAs type materials. Thus, it has been proposed to introduce a passivation layer by leaving, on the surface between the emitter and the base, a semiconductor material with a wide gap that is very thin and therefore depopulated. This prevents the appearance of recombinant electrical defects at the base/passivation interface. More specifically, FIG. 2 describes a structure in which the emitter E has a special architecture obtained by the partial etching of a layer of GaAlAs deposited on a layer of GaAs constituting the base B. Two contacts CB.sub.1 and CB.sub.2 are made on the emitter layer on the surfaces S'1 and S'2. Then, by an appropriate treatment, these contacts may get diffused on the thickness e.sub.0. In this configuration, the electron/hole recombinations in the base, which previously enjoyed favorable conditions at the free surface of the base, are limited owing to the emitter thickness e.sub.0 maintained above the base. The problem of this type of structure however lies in their poor stability inasmuch as the diffusion achieved for the contacts cannot be entirely controlled and may therefore continue to undergo changes with time and temperature.
This is why the invention proposes a heterojunction bipolar transistor in which the surface recombinations are minimized through electrically insulating elements located on a part of the base and in direct contact with the flanks of the emitter mesa.
The invention can be applied to structures in which the mesa is an emitter mesa as well as to structures in which the mesa is a collector mesa.